Polymers such as rubbers and resins in their original state are susceptible to oxidative deterioration by means of heat or the like. Therefore, in order to enhance their heat resistance, various aging inhibitors are added thereto, and thus heat resistance is obtained for purposes. Well-known aging inhibitors include phenol-based aging inhibitors and amine-based aging inhibitors, and a representative class of the amine-based aging inhibitors is diphenylamine-based aging inhibitors, which are diarylamine compounds.
As the diphenylamine-based aging inhibitors which are diarylamine compounds, compounds such as shown below are described in Japanese Patent Application Laid-Open (JP-A) No. 9-53070 (Patent Literature 1), JP-A No. 10-298551 (Patent Literature 2), JP-A No. 11-21411 (Patent Literature 3) and the like, and those compounds are used for general use.

In recent years, polymeric materials such as rubbers are increasingly used in high temperature conditions which are severer than in the past. To take an example of a rubber used in the periphery of an engine of an automobile, there is a tendency that the temperature inside the engine room increases due to the increased power output of automobile engines, or the emergence of low pollution engines. Thus, rubber materials that are used in the periphery of such an engine are required to have a heat resistance property higher than conventional rubber materials, and therefore, there is a strong demand for a rubber material which can endure in that environment. As one of the means to achieve the purpose, there is a strong demand for a diphenylamine-based compound having a new structure, which does not cause oxidative deterioration of polymers such as rubbers and resins even if used in a higher temperature environment than in conventional cases, and is appropriate for aging inhibitors having high heat resistant effect. However, sufficient effects could not be obtained with diphenylamine-based aging inhibitors that are conventionally known.
Among rubber materials, an acrylic rubber is known as a rubber having excellent oil resistance, particularly oil resistance at high temperatures, and having satisfactory heat resistance, and there is an increasing demand for an acrylic rubber for hoses, oil seals, gaskets, O-rings in automobile-related fields, and for conveyor belts mounted in apparatuses and machines. For the rubber members for use in automobiles, particularly the rubber members in the engine rooms, performance enhancement of superchargers (turbochargers) along with an increase in the output power of engines, and the recent tightened regulations on exhaust gas have caused a further demand for an enhancement of the heat resistant performance.
For example, it is disclosed in JP-A No. 11-21411 (Patent Literature 3) described above that when an acrylic rubber and two kinds of diphenylamine-based oxidation inhibitor are used in combination, the heat resistance of a crosslinked acrylic rubber product is enhanced. However, when this technology is employed, improvements in the tensile force change rate, the elongation change rate, and the compression set in a short-term heat resistance test are recognized, but the effect in a long-term heat resistance test of the crosslinked acrylic rubber product at high temperatures is not recognized.
Furthermore, in recent years, it has been suggested to use various aging inhibitors instead of 4,4′-bis(α,α′-dimethylbenzyl)diphenylamine or the like, in order to enhance the heat resistance of crosslinked acrylic rubber products. For example, it is disclosed in WO 2006/001299 (Patent Literature 4) that the heat resistance of a crosslinked acrylic rubber product is enhanced by incorporating a styrenated diphenylamine compound into a carboxyl group-containing acrylic rubber. However, when this technology is employed, an improvement in the tensile strength change rate is acknowledged compared to the case of incorporating 4,4′-bis(α,α′-dimethylbenzyl)diphenylamine; however, improvements in the elongation change rate, the compression set, and the effect in a long-term heat resistance test of the crosslinked acrylic rubber product at high temperatures are insufficient.